Tornadoes make the news most often in spring and summer, when they commonly tear through towns and cities. But these violent storms don't really follow any set season, as we noticed last month when an spate of unseasonable storms struck the Midwest. Tornadoes are instead driven by local weather patterns, when warm, humid air, full of energy, meets an area of colder, fast winds. Because they are so driven by weather, it makes sense to wonder just how much changes to climate on a regional or global could change the frequency of these local, deadly weather systems.

At a press conference at the American Geophysical Union conference in San Francisco on Tuesday, James Elsner, a geographer at Florida State University, posed the question: Are tornadoes getting stronger? The answer: Maybe, but we need a lot more information to know anything for sure.

Elsner said that the title of his presentation was "necessarily provocative." Necessary, in this case, to get people talking about how severe local storms are related to climate and, beyond that, to climate change.

"I think there are a lot of people out there who won't want to think about the question," Elsner said. Tornadoes, while exciting, aren't exactly the most pleasant food for thought. Their destructive paths can level towns, uprooting the lives and livelihoods of people across the country and around the world.

Elsner said that what he really wants is for the tornado community and climate community to talk and perhaps agree on what data sets could be used. To that end he's organized an International Summit of Tornadoes and Climate Change for next spring in Greece.

Unfortunately, that kind of collaboration might be difficult to forge. During the past few weeks, an ongoing feud among scientists arguing over how tornado rates and intensity relate (or don't relate) to climate change raged in the pages of The New York Times. It started when one scientist from UC Berkeley wrote an op-ed claiming that the number of severe tornadoes has decreased in recent years and that the trend could be due to climate change. Then a group of other scientists penned a letter to the Times and LiveScience saying that record-keeping on tornadoes is so bad that it is nearly impossible to make any such claim about tornadoes and climate change. The debate continues in the comments on Andy Revkin's blog.

Elsner looked at data from the National Oceanic and Atmospheric Administration's Storm Prediction Center, which has information on tornadoes going back to the 1950s. Elsner narrowed down the data by looking explicitly at storm data between 1994, the first time the United States was almost completely monitored by radar, and 2011, the last full year of available data. He then looked at the damage paths, or the lines of destruction left in the wake of a tornado. He took the data on the length and width of these scars in the earth and correlated those measurements with the ratings of the tornadoes that went with them.

Tornadoes are rated on the Enhanced Fujita (EF) damage scale, which goes from 1 to 5, with an EF5 tornado doing the worst damage. Elsner unsurprisingly found that the stronger tornadoes had longer and wider damage paths. He also found that the average length, width, and presumed wind speed of the tornadoes in the data set rose in all categories over that 17-year period. But he stopped short of claiming that any change was connected to climate, saying scientists need more data on that.

Just studying tornadoes to get this raw information is no picnic. Wind speed, one of the most direct measurements of a tornado's strength, is notoriously difficult to obtain. For an accurate and direct reading, a mobile radar station has to be near a tornado when it forms, and there aren't a whole lot of those on the ground.

The width and length measurements don't have the same kind of time constraints, but damage paths stretch for miles, and the National Weather Service has limited ground survey staff. On their website, the Storm Prediction Center says that there are no rigid criteria for storm events that call the NWS out to do a survey.

In addition, tornado reporting has long suffered from population bias—the fewer people living in an area, the less likely a tornado will be reported. Elsner pointed out that the rate of population bias in tornado reporting has been decreasing over time as more of the U.S. is more heavily populated. That difference makes it more difficult to extrapolate historic storm events into present trends.

While he stayed away from explicitly linking the observed longer, wider swaths of tornado destruction to climate change, Elsner did say that there have been increases in humidity across the Plains, and that the increasing trends of tornado strength are most pronounced farther south. This is key, because tornadoes thrive on warm, moist air as an energy source, and increasing amounts of humid air from the Gulf of Mexico could, theoretically, result in more storms—if conditions were right.